The invention relates to the general field of powder injection molding (PIM) with particular reference to ways to manufacture structures having complex shapes.
The traditional methods of molding parts, for example plastic or composite parts, by vacuum molding, sheet compound molding, reaction injection molding, injection molding and/or rotational molding require expensive, complex and specific tooling in order to form a particular molded part shape. If a designer chooses to include features such as undercuts in the molded part, expensive, complicated collapsible tooling is required to remove the tool from the molded part in the area of the undercut. Such expensive, complicated tooling generally must include a complex system of slides and ways to remove the tooling from the formed part. Even if the undercut is permitted at molding, the shape and complexity are very much limited.
Some undercut or internal features can also be molded by introducing gas under pressure, as in blow injection molding. While this does not require complex tooling, again the shape and complexity of undercut or internal feature is limited to simple geometries The new technology of powder injection molding, can likewise require expensive tooling, depending on the complexity of the part and the number of cavities. To include undercut features in the molded part requires complicated tooling with slides and ways.
Even if such methods are to be employed, the complexity of the undercut is very limited. Manufacturers have to purchase or construct very complex tooling in order to form simple undercuts in their molded parts. Otherwise they simply have to forego such undercuts and process the molded parts into the sintered parts. This means that expensive secondary operations must be added to the process to introduce the undercut features into the sintered parts. Again, there is a limit to the complexity of the undercuts and, most often, such operations are time-consuming and have low production output.
A routine search of the prior art was performed. The following references of interest were found. All are concerned with methods to form or remove binders and feedstock:
Zhang et al. U.S. Pat. No. 5,332,543, xe2x80x9cMethod for producing articles from particulate materials using a binder derived from an idealized TGA curvexe2x80x9d; Zhang et al. U.S. Pat. No. 5,415,830, xe2x80x9cBinder for producing articles from particulate materialsxe2x80x9d; Peiris et al. U.S. Pat. No. 5,397,531 xe2x80x9cInjection-moldable metal feedstock and method of forming metal injection-molded articlexe2x80x9d; and Zhang et al. U.S. Pat. No. 5,401,462 xe2x80x9cRemoval of binder for producing articles from particulate materials by use of a specific TGA curvexe2x80x9d.
It has been an object of the present invention to provide a process for manufacturing a metal/ceramic article, including undercut or hollow features, which does not require the use of expensive and complicated tooling
Another object of the present invention has been to provide a process for manufacturing complex internal undercut features without the use of costly secondary operations on the sintered parts.
A further object has been that said process be well suited to mass production and be economical to use.
These objects have been achieved by the disclosure of a process in which the shape of the undercut/hollow feature is initially molded using a disposable material such as a degradable polymer. The PIM feedstock is then molded onto this to form the required shape geometry, in effect encapsulating the polymeric feature by the PIM feedstock. The resulting two-material part is then sent for processing which removes the polymer through solvent or thermal process. The binder inside the PIM feedstock is then also removed through either solvent or thermal processes. After the polymer and the binder have been removed, the part now comprises a powder skeleton that contains the internal undercut feature within itself After sintering the result is a metal/ceramic part with internal undercut feature. The technical advantage of the present invention is that it does not require complex toolings or costly secondary operations while retaining the flexibility to design any internal undercut features of complex geometry. An additional embodiment of the invention is also disclosed in which a solid structure is encapsulated inside a hollow shell said structure being free to move around inside the shell.